Servo systems are classified according to the parameter being controlled. For example, rate servos control motor rate to a given profile, position servos control angular position of the motor's shaft, and torque servos control motor torque to a commanded level.
In applications where servos are required to control electric motors that drive output shafts through reduction gears, rate servos are preferred over position servos. In general, rate servos provide smoother control than position servos. The difference in control is attributed to an integrator in the position servo. The integrator integrates angular velocity to obtain angular position. However, it also smoothes variations in angular velocity.
Despite providing smoother control, the rate servo cannot hold the commanded position of the output shaft under certain conditions. For example, once the output shaft has reached its commanded position, it can be backdriven by external loads. Backdrive generally occurs at low speeds. Because rate sensors in the rate servo cannot provide accurate measurements at low rates (e.g., a few hundred rpm), the displacement in angular position due to backdrive cannot be adequately corrected.
Backdrive can be prevented by mechanical devices such as clutches, no-backs, brakes and gearboxes having low mechanical efficiency. Once the output shaft has reached its commanded angular position, the mechanical device is enabled.
However, each of these mechanical devices has certain drawbacks. Low gearing efficiency results in greater wear and shorter life of the gearbox. Mechanical clutches and no-backs cause an increase in complexity and weight of the servo design while causing a decrease in reliability. Brakes are subject to wear. In applications requiring numerous repeated operations, the braking surfaces of the brakes must be replaced frequently.
Alternatively, backdrive can be prevented by a position servo used in conjunction with the rate servo. In general, position servos provide greater accuracy than rate servos in stopping and holding the motor at a commanded position. The resulting hybrid servo system would initially select the rate servo to slew the motor to its commanded position (rate mode), and then it would select the position servo to prevent backdrive (position mode).
Switching between the rate servo and position control can be performed by an on/off controller.